1. Field of the Invention
This invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device comprising a dynamic scattering liquid crystal cell formed by a nematic or long-pitched cholesteric liquid crystal orientated in a twisted pattern between a pair of substrates carrying transparent electrodes thereon, and polarizers.
2. Description of the Prior Art
Recently, matrix type liquid crystal display devices capable of displaying many types information, have begun to draw more attention than conventional segment type devices. However, matrix type devices have encountered problems. While they can display an image having improved resolution with an increase in the number of lines, or electrodes, the increase in the number of the electrodes, particularly the scanning lines, leads to a reduction in the duty, i.e., the time for which a signal is applied per line, and therefore, in a cross-talk margin. If a liquid crystal is used as a display element, a satisfactory contrast is not available because of the dull transmission characteristics and slow response.
The following expedients have been proposed to solve the aforesaid problems:
(1) To develop a liquid crystal material having a sharp threshold, and employ a mode of liquid crystal performance showing sharp transmission-voltage characteristics;
(2) To optimize a matrix addressing system to enlarge a driving margin (.alpha.=V.sub.on /V.sub.off); and
(3) To improve the electrode and panel construction to get a seemingly higher resolution.
This invention is directed to the mode of performance as referred to at (1) above. The DTN mode utilizing the light depolarization feature of the DS mode having a twist angle of 90.degree. has recently come to attract special attention. It is reported that The DTN mode is characterized by a sharp rise in the vicinity of the threshold value and a wide viewing angle, and is suitable for a matrix display device having a large number of scanning electrodes (Tatsuo Uchida, Yutaka Ishii and Masanobu Wada: "Properties of a Display Device Using Depolarization in a Twisted Nematic Liquid-Crystal Layer (DTN-cell)", Proceedings of the SID, Vol. 21/2, 1980, pages 55 to 61).
As opposed to other kinds of display such as of characters, however, the display of a TV picture requires a halftone, and the device must be capable of responding properly to a particularly high frequency component of an image signal arising from pulse width modulation. Moreover, it is important that the proper response can be maintained for a long time.
In view of these problems, the inventors of this invention have found it useful to incorporate a specific dopant into a liquid crystal layer, and apply a specific sealing structure to a liquid crystal cell. As a result, they have succeeded in obtaining a liquid crystal display device which is suitable for the display of an image signal, and which constitutes this invention.